1. Field of the Invention
The present invention relates to a digital-analog converter, and more particularly to a digital-analog converter suitable for use in converting digital audio data into an analog signal.
2. Description of the Related Art
In compact disc players (DC players) or digital tape recording/playback devices (DAT devices), digital audio signals are converted into analog signals prior to output from speakers.
In commonly employed digital-analog converters (hereinafter referred to as DA converters) for music playback, digital data is converted into a direct current and the direct current is converted into voltage at every sampling period to be held. The held voltage is shaped by a low-pass filter into a continuous, smooth analog signal before being outputted. The foremost problem encountered in such DA converters as mentioned above is phase distortion ascribable to the low-pass filter, causing a deterioration in sound quality.
To overcome the above problem, the present and other inventors have proposed a DA converter shown in FIG. 10 in Japanese Patent Application No. 62-78878 corresponding to U.S. Pat. No. 4,862,170. This DA converter includes a digital data generator 10 for generating digital data at every predetermined time T, a digital data storing section 11 comprised of shift registers (11.sub.-4 through 11.sub.+4) for storing, by successively shifting, m items of the latest digital data V.sub.-4 through V.sub.+4 generated at every predetermined time T, a unit pulse response signal generator 12 comprised of partial signal generators (12.sub.-4 through 12.sub.+4) for dividing a unit pulse response signal SP (see FIG. 11) at a predetermined time interval T and for repeatedly generating at every time T m partial analog signals S.sub.-4 through S.sub.+4 (see FIG. 12) which have been divided, a multiplier 13 having multiplying-type DA converters (13.sub.-4 through 13.sub.+4) for multiplying partial signals Si by predetermined digital data Vi stored in the shift registers corresponding to the partial signals Si, and a mixer 14 for outputting an analog signal by combining the output voltage of the multiplying-type DA converters.
According to the above-mentioned method, without utilizing a low-pass filter, a pulse response signal permits smooth compensation for a gap between digital data at a period T, thereby generating a continuous analog signal that is free of phase distortion.
There are, however, problems in the DA converter shown in FIG. 10 in that it requires a digital storing section, m analog partial signal generators, m multiplying-type DA converters, and analog mixers, causing largeness in size, an increased number of parts, and it is not suitable to miniaturize the converter. Further, because of the largeness in size, an increased number of parts and special IC chips for the multiplying-type DA converters, the DA converter becomes considerably expensive.
Furthermore, since there are a number of adjusting points because of changes in volume in the multiplying-type converters and analog partial signal generators, it requires skill to adjust the points. It is troublesome to carry out re-adjustments caused by deterioration with age and to re-adjust the points when unit pulse response signal waveforms are altered.
Moreover, the adjusting points deviate with temperature, and it is far more troublesome to adjust the points strictly in accordance with temperature.
In addition, since a number of parts, with a wide range of variability, such as a capacitor, are utilized in the partial signal generators and the multiplying-type DA converters, variability is likely to occur. This leads to a problem in that extremely small spike-like noises are imprinted onto an analog signal.